how does a CAR's AC regulate temperature??

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285
Location
nyc
i have observed that most mount-on-window home AC's regulate their temperature shutting itself on and off--this is why i always choose the lowest temperature which keeps the AC on at all times, i feel this is its true operating mode. yet with my car, it truly is able to achieve the temperature you set it to. i wonder how it does this. any ideas?
 
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25,045
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ON, Canada eh?
There are a bunch of different ways this can be controlled. Some use High pressure switches, some use an ECU to control it using a temperature sensor, and some use a thermal sensing unit that cuts out the compressor.
 

Bill in Utah

Staff member
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UT
And some blend the heat and a/c to make the temp that you select... IE they are dumb. Turn it on and it cycles. It will cycle the same no matter how hot or cold the temp is set for. Bill
 
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Saskatchewan
I don't think cars with a belt-driven compressor can vary compressor speed or output, so it must be either cycling the compressor or blending warm air to achieve the desired temperature.
 
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md
My 96Q45 uses a variable displacement compressor-pretty exotic for its time,now the technology is more common on moderately priced cars- compressor never cycles off so you avoid the change in idle speed/power output normally felt as a compressor cycles.
 
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North Carolina, USA
[quote=qship1996]My 96Q45 uses a variable displacement compressor-pretty exotic for its time,now the technology is more common on moderately priced cars- compressor never cycles off so you avoid the change in idle speed/power output normally felt as a compressor cycles. [/quote This is used on some Lexus rides as well and is not the best method in that compressor failures are much higher and the compressors are rediculously priced. Most Asian rides still use the simple thermister on the evaporator coupled to a A/C control module. Not rocket science but it does work reliably.A/C techknowledge is at it pinnacle on Asian rides but it has come at the price of reliability and durability. It was nothing for an older Corolla to have 300K on it and the air would run 28-35 degrees at the vent never having even freon added to it. Ditto for the older Nissans that had the Calsonic supplied air systems.We just did not do a/c repairs on older Maximas,Hardbody trucks ect.. that had the simple systems. Calsonic was an excellent supplier. Denso has had some compressor failures on Lexus and upscale Toyota/Honda applications because the customer generally ran the climate control constantly which meant the compressor ran all the time even in winter. When we purchased our last new Honda I intentionally got one that did not have automatic climate control on it. As a life-long Honda/Toyota/Nissan tech I saw much better longevity from the standard a/c systems. It was the same in spades when I got my start working on domestic cars. The increase in head pressures that came with 134-A and the neccessity of the compressor running almost constantly on Automatic climate systems has put many comressors in an early grave. Some compressors now cost more than what you can buy a complete engine for the same vehicle. That's stickin to the poor old workin'man. I did an a/c repair on an older Q-45 that was near 3 grand when all was done.Same on an LS-400. Most of it you can thank the big-brother mentality that seems to be so vogue at this time.
 
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3,735
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Miami-Dade County
 Originally Posted By: Bill in Utah
And some blend the heat and a/c to make the temp that you select... IE they are dumb. Turn it on and it cycles. It will cycle the same no matter how hot or cold the temp is set for. Bill
I read in my local newspaper [if you have auto temp control] if you keep your car temperature set at 77 instead of 70 it will be less strain on the car and burn less gas. Sounds like alot of B.S. to me.
 
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6,902
Location
Louisiana
I'm glad the Fit has a pretty simple unit. When I cool off, I just hit the a/c button and keep it on recirc. Saves me about 2-3 mpg.
 
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740
Location
md
Original variable displacement factory compressor lasted 200,000 miles on my Q45....replaced it with exact same factory brand new identical part which cost about $600. Not outrageous considering the lifespan and zero other a/c problems since brand new?
 

Win

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4,705
Location
Arkansas
Variable displacement compressors seem to be the norm these days. Our '04 Xj8, my previous '04 GTO, and my '08 G8 all have variable displacement compressors. Pretty sure the '09 Solstice does as well, I've never felt it cycle.
 
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Location
Saskatchewan
 Originally Posted By: Mokanic
This is used on some Lexus rides as well and is not the best method in that compressor failures are much higher and the compressors are rediculously priced . . .
Interesting stuff. I actually modified the circuit board on my basic climate control unit so the A/C never comes on automatically, which it did in all positions except dash and dash/floor. I did it for mileage and because the air is dry enough already here, but it sounds like it may increase compressor life as well. I try to run it at least once a month.
 
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8,859
Location
Texas
 Originally Posted By: CROWNVIC4LIFE
I read in my local newspaper [if you have auto temp control] if you keep your car temperature set at 77 instead of 70 it will be less strain on the car and burn less gas. Sounds like alot of B.S. to me.
It is generally [censored]. Things are changing in how auto AC works right now with the growing use of electric compressors (especially in hybrids, but they're coming for other apps too). But if you are talking about cars made prior to 2009, 99.9999% of them work as follows: 1) The AC system tries to keep the evaporator coils *just* above freezing- about 33-35 degrees F regardless of what speed the fan is running or what you set the cabin air temperature. It does this one of two ways- method number one is by throttling the refrigerant flow on the vacuum side (ie starving the compressor for return flow, which "unloads" the compressor and makes it draw less power from the engine). This is called "evaporator pressure regulation" or an EPR type system. It requires a thermal expansion valve to work with the EPR valve, so its a little more expensive. The second and much more common method is to use a thermostat embedded in the evaporator coil to turn off the compressor when it gets down to about 33F and turn it back on when the temp rises to about 35F. It can use either a fixed orifice tube or variable orifice valve instead of a thermal expansion valve, so its cheaper. 2)To regulate CABIN temperature, a portion of the air that has been chilled by the evaporator coil is directed through the heater core and then blended back into the cold air stream. This has the advantage of always de-humidifying the air which helps it feel cooler. The slower you run the FAN speed, the less the compressor runs and therefore the less fuel you use. That's because its fan speed and ambient air temperature that determines how long the compressor runs before it gets the evap core to 33F and can shut off. Of course in the summer it probably runs 100% of the time and can't ever get to 33F in the heat of the day.
 
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ME
 Originally Posted By: Mokanic
It was nothing for an older Corolla to have 300K on it and the air would run 28-35 degrees at the vent never having even freon added to it.
How are you getting below freezing at the vents? I have a fancy CCOT comopressor in my econo saturn still blowing cold air at 206k. The best part, if it's too cold (rarely), when one moves the slider towards warm most cars bleed in some heat from the heater core, not cutting the drag from the engine any.
 
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